The long term objective of the proposed work is to identify naturally occurring mosquito mechanisms that affect the development of Plasmodium falciparum in the vector, Anopheles gambiae, in West Africa. These mechanisms may include vector immune responses as well as variant molecules involved in critical vector-parasite molecular interactions. Understanding these mechanisms will illuminate the biology of the relationship between vector and parasite and could be useful in developing a novel malaria control strategy. The specific aims are to: 1) Identify the major An. gambiae genetic loci that control susceptibility to P. falciparum in the natural malaria transmission system in Mali, West Africa. A genome-wide scan will be conducted in large pedigrees of wild An. gambiae. Loci will be detected by genetic linkage of midgut oocyst number with a multilocus microsatellite genotype at 10 centiMorgan resolution. 2) Analyze the same mosquito pedigrees to determine the influence of two known refractory mechanisms, encapsulation and lysis, in the natural P. falciparum transmission system, by genetic linkage analysis with microsatellite markers. In addition, a laboratory study will determine the parasite species-specificity of the lysis refractory response of An. gambiae, in which parasites are destroyed within midgut epithelial cells. 3) Determine the association between chromosomal forms of An. gambiae and susceptibility of P. falciparum infection, by cytogenetic analysis of the infected mosquito pedigrees. Natural populations of An. gambiae are subdivided into genetic subgroups based on chromosome inversions. Preliminary evidence suggests the forms may differ in malaria vector competence. Laboratory studies have shown that vector mechanisms with a simple genetic basis can greatly affect the efficiency of parasite development. However, laboratory systems can depart from the biology of natural malaria transmission in important ways. Mechanisms of vector susceptibility to malaria parasites in nature have not been studied. This project will be the first examination of naturally-occurring mechanisms of mosquito refractoriness to malaria parasites in the P. falciparum transmission system.